Mixture control
 

Not discussing about how to lean mixture, but on something else:

1. As per POH (eg. C172), leaning is done when power is less than 75%. Let's say cruising at 5,000ft, with throttle full open, power is 70%. Can you lean? Throttle is full open! With throttle full open, isn't there a "fuel enrichment" function which adds slightly extra amount of fuel with throttle full open for cooling? Won't you be leaning, but fuel enrichment is "riching" it again...?

2. While set up (properly leaned, fix pitch prop) cruising at say 5,000ft, you fly into an area of slightly different ambient conditions causing RPM change & also airspeed change. Is it ok to leave the mixture lever as it is and adjust throttle to regain your required airspeed?

The fuel enrichment function works mechanically based on throttle position if the engine is equipped with such carburettor/injection system. Basically, instead of increasing fuel flow linearly with moving the throttle forward, the fuel flow starts to increase "exponentially". Try to imagine you have two valves for setting the fuel flow (I know this isn't the case for carburetted engines, just trying to explain). The first one is controlled by throttle and the second is controlled by mixture. For example: you set the throttle to desired position and the first valve gives you 10 USgal/h fuel flow. If you leave the mixture full rich, the second valve is fully open and the fuel flow remains unchanged, but if you pull the mixture towards the lean, the second valve starts to close and reduces the fuel flow coming from the first valve, so the enrichment function on the first (throttle-controlled) valve doesn't have influence on the second (mixture-controlled) valve. For example in Arrow II, the full throttle fuel flow at MSL (ISA) is 16 USgal/h and using mixture you can achieve fuel flow to the engine anywhere in range from 16 (full rich) and 0 USgal/h (idle cut-off).

In a normal C172, equipped with Lycoming O-320 you will get much more than 70% at 5000ft pressure altitude (ISA conditions)! As per my tables for C172N (O-320-H2AD), the engine produces 75% power at 8000ft pressure altitude, ISA conditions and 2650 rpm (fixed pitch prop). All this is at "Recommended Lean Mixture", which is about peak EGT (manual says 25-50 RPM below peak RPM on the lean side). Nobody is saying you can't lean above 75% power - you have to do it in the climb if you want achieve satisfactory performance, but you can't lean it to peak RPM (best power), since the CHTs would head for the sky.

While most instructors will tell you that every power adjustment (increase as well as decrease), requires full rich mixture, this just isn't true. If you have to increase power a bit (not full throttle of course) for let's say 10 seconds, just leave the mixture where it is and reduce the power to cruise RPM when you don't need the extra power anymore. As far as reducing power goes, there's absolutely nothing wrong with reducing power while leaving mixture intact - and if the engine starts running rough (and it probably won't until you go near idle), just enrich the mixture until it's smooth again.

A very good point. Unfortunately most instructors treat the mixture control as an engine on off switch with only two positions, full rich and ICO, and don't teach there students how to lean. The fact is the engine should be leaned whenever you are below 75 % power regardless of the altitude. I also leave the mixture leaned on descents, maybe enrichening slightly if it is a prolonged descent. The mixture only goes full rich on the prelanding check and not even there, if I am operating from a high elevation airport.

Confusion exists from folk lore spread by 'teach by numbers' instructors who do not keep themselves up to date. Leaning should always be carried out with reference to the aircraft or engine manufacturers instructions. This, below, is for normally aspirated Lycoming engines

Various Lycoming Flyer articles have emphasized proper leaning at the manufacturer’s recommended cruise power. Before delving into the savings to be obtained by leaning, it may be appropriate to again review those factors that affect leaning at cruise.
First, we must know that cruise power for Lycoming normally aspirated engines is generally considered to be 55% to 75% of the maximum power for which the engine is rated. At these power settings, the engine may be leaned at any altitude. There has been confusion about the reference to not leaning below 5000-feet density altitude. Remember that this reference only applies to those power settings above the cruise range — those normally used for takeoff and climb. Once cruise power has been set, leaning to best economy should be standard procedure as damage to the engine will not occur from leaning at cruise power settings.

FS - You should be referring to density altitude not pressure altitude for leaning

I was trundling home the other night in an AA5, about FL50, decided that I needed to climb to FL90, mixture rich, full throttle - 300fpm.

I then remembered that when I did a high altitude flying course in Arizona, we leaned for best power. Leaning a little, leaving the speed alone, I got 550fpm and was at FL90 in a reasonably sensible time.

One forgets these things, and if I'd not remembered that on this occasion I'd never have made FL90 and been forced to return home at about 2000ft, dodging cloud and ATZs.

But leaning isn't just for efficient cruising, it can be for best power at elevated density altitudes as well, even when it's about ISA-10, as it was that night.

I admit however, that I usually find it most convenient when changing flight condition to go fully rich, then lean again to the condition I want.

G

While this comment may (or may not) be correct for the C172, it is certainly not true for the general case. At the very least, it will depend (for normally aspirated engines) on the carburettor / engine combination.

For example, the Teledyne Continental Manual for the C75, C85, C90 & O-200 series of engines reads as follows:

7-7. LEVEL FLIGHT
...
During flight, observe the following procedures and precautions:

a. If a Stromberg carburetor is installed leave mixture control in the "FULL RICH" position at altitudes less than 5000 ft above sea level. If a Marvel-Schebler carburetor is installed the mixture control may be adjusted to obtain best rich power at any cruising altitude.
...

Without wishing to be controversial ;), I would suggest the safe course is to follow the procedure in the POH or Engine Manual.

JD
:)

Thanks, but if you look at my post thoroughly, you will see I added ISA conditions where pressure altitudes are mentioned. I did it just to make it more understandable to OP in case he goes looking in Cessna's manual which is in form of a table with pressure altitudes and ISA, ISA+20 and ISA-20 columns.

Otherwise I agree, air-to-fuel ratio is mass of the air in the cylinder divided by mass of the fuel in the cylinder, which is actually air mass flow divided by fuel mass flow. And air mass flow depends on air temperature and pressure, so it is in fact the density altitude which is important - and most manufacturers use density altitude in performance tables/charts.

First off my statement to "always" lean the engine is 100 % correct as a response to the original post who was asking about leaning a C 172. There are no C 172's that should not be leaned in cruise.

Your comment about the Stromberg carb is interesting. Personally I have never seen a Stromberg carb as every C 65-75-85 engine I have ever flown has had a Marvel-Schebler carb installed. Any aircraft with a stromberg carb is going to be so old it will not have a POH as we currently understand the term to mean.

However many, if not most, pilots and owners do not seem to know that both Lycoming and Continental publish their own operating manuals for pretty much every engine they have ever made. The engine information in the POH will have been derived from these manuals but engine manufacturers manual will go into much more detail than the POH. Every aircraft owner IMO should have a copy of the engine manual of their aircraft in their library. For renter pilots most 4 place rental machines will have a version of the ubiquitous Lycoming 0 320, so that manual is a good investment

Correct, unless its a Beagle Pup!

I operated a C90 with a Stromberg for a few years and was left somewhat disappointed by the 'information' generally available on the net.

A Stromberg leans perfectly well - once you understand what it is doing and how that works. It is nothing like so immediate as a Marvel - you need to let the new float chamber level/pressure 'settle', but with a simple single probe EGT and CHT it was very obvious what was happening.

Once I had that sorted, I leaned it all the time - just like a Marvel, but you have to be much slower and smaller making the adjustments and waiting for their effects.

But with all these engines once at any decent altitude leaning is essential. That aircraft went to 12,000 a few times and without leaning there is no way it would have got there!

What about a change in airspeed while cruising at the same altitude?

Example:

Cruising at 5,000ft, properly leaned (LOR), 1800RPM, 100kts.
Now, at same altitude, you want to increase speed to 130kts, which would require about 2200RPM (this is just an example!).
What do you do with the mixture?

a. Mixture full rich, power increase to 2200RPM or so as required to maintain 130kts, then lean (LOR).
b. Power increase to 2200RPM or so as required to maintain 130kts, then mixture full rich, and then lean again to LOR.
c. None of the above (then pls give an anwer...)

In the real world, c. None of the above.

Just rich it up a little before you increase the power. It's nice to avoid going too lean, even for brief periods, so a bit of rich assures that. Going full rich at altitude is not necessary, or ideal. If you're way up there, full rich will make it run rough, until you lean it again.

Though a well set up fuel injected engine can be leaned with precision, a carburetted engine really cannot. Any leaning will be a compromise. Temperature scanners help. With one, you will probably see that only certain power settings give you anything close to equal fuel distribution across all of the cylinders.

Leaning is nice, and appropriate at higher altitudes, but otherwise, the cost saving and "good" it does to lean an engine, can be quickly turned into costly bad, if you go too lean for too long. Perhaps for reasons of perceived economy, coupled with control and opportunity to fiddle with knobs, fixed wing pilots tend to lean in flight. The same piston engine series are generally not leaned in flight when they are installed in a helicopter. Yes, helicopters don't often fly as high as fixed wing regularly do, but the training, and flight manual usually say to leave it full rich.

Don't get me wrong, I do lean all the time in fixed wing aircraft, but I don't get carried away about that final precision of lean, unless it's a very long flight leg, and optimizing range becomes a factor.If you're running 100LL, and your exhaust is whitish grey, you did fairly well.

PilotDAR,

1. "Just rich it up a little before you increase the power",
but wouldn't it be better if after this new power setting is done, to "reset" the "system" - that is, with this new power setting, you don't know if you're still at eg. 50RPM lean of peak.
So "reset" by mixture full rich, then lean until 50RPM lean of peak, just to be sure...

2. Basically what I'm concerned with is having set up at eg. 50RPM lean of peak, then playing around with the throttle at the same altitude (& ambient conditions), will it be always 50RPM lean of peak at every other throttle setting?

3. Another concern, eg. I am cruising at 5,000ft, 2200RPM, full rich, 100kts. Then I lean, get a peak of 2300RPM, so lean another 50RPM. Now I am flying at 2250RPM, properly leaned, but due to higher RPM now (due to proper leaning) airspeed is 110kts. I want 100kts. Is it ok to just adjust the power as required to get 100kts?

This site is quite an eye opener: Pelican's Perch - More Stories

Following on from that...

What do you mean by 'too lean'? that doesn't appear to make much sense without some form of context.

Yes; that is basically correct.

If you set the mixture to around say peak EGT, then you can vary the MP quite a lot and the engine will remain around peak EGT.

The exception is when there is a big altitude change e.g. in a descent from FL150 to 3000ft you will need to enrich the mixture, because if you were at peak EGT at FL150, and then descend, the engine will go leaner and leaner and long before you are at 3000ft it will be way LOP and will probably start running rough.

I have a writeup on non-turbo fuel injected engine management here. It is quite long but you will get the general idea.

I don't bother with LOP; it does not produce any more MPG, except as a side effect of delivering less power and flying slower as a result of that ;) And even with GAMI injectors my engine does not sound that smooth when LOP.

Regarding the original Q, it is correct that one should not lean a Lyco motor when the power setting is above 75%. However, this is a crude guideline because, following takeoff, the power drops continuously as one climbs, and it is obvious that there is nothing inside the engine which will suddenly wake up at 75% and blow it up :) The constant-EGT climb technique is generally accepted as totally safe, and it delivers an easy method for climbing all the way to one's operating ceiling without touching anything but the mixture lever.

What is really important is setting a reasonably high IAS ASAP after takeoff, to avoid exceeding ~400F on any cylinder. For me (TB20) that means trimming for 120kt pretty soon. And sometimes it cannot be done; for example I departed from some place in Turkey at +35C and could not keep it below 430F, but that is still OK. Some SID profiles also make it hard - look up some of the ones in the Alps, for amusement :) If a SID has a 10% climb profile then it is probably a no-no for piston ops.

True. If you have proper instrumentation and an injected engine (preferably with matched injectors like GAMI) you can do a lot more, including leaning above 75%.

Some friends of mine fly a Sportscruiser with full engine instruments and an injected engine. They lean as soon as they're off the ground, based on a target CHT of 375F I believe. (Of course leaning for any CHT requires patience - a few turns of the vernier mixture control at a time, and then 60 seconds wait for the temperatures to settle.)

Only when in the cruise do they properly lean for best economy using the EGT.

But for those of us that fly a non-turbo, carbureted engine, a fixed pitch prop, and no more instruments than the RPM, a very crude single CHT and the Mk. 1 eardrum, leaning above 75% power is not worth the risk of cooking the cylinders. Below 75% lean for best RPM (best power) or slightly richer than the point where the engine starts running rough (best economy).

I agree.

Should also mention that the constant-EGT method involves hardly any leaning at low levels. The mixture lever comes back just a little bit initially.

I would not use a constant-CHT method. The CHT will depend a lot on the airflow, and will obviously be directly related to the ambient air temperature. And anybody doing that probably has just a single CHT gauge so other cylinders could be running too hot.

I see a lot of exacting numbers in this discussion, and description of very specific techniques. Other than lean of peak, I'm not suggesting that any of them are not worthy of consideration, but I am suggesting that for leaning Lycomings, and Continentals, particularly those with carburetors, one technique is not universally appropriate. Indeed, a technique found to be excellent on one engine, could verge on being harmful on another of the same model. There are too many variables for a one size fits all leaning technique.

Certainly, if you have a well instrumented engine, and a lot of experience with operating it (peterh337 for example), you can have the technique down to a fine art, and get excellent results. If, however, you're going from one to the next in a rental fleet, and the instrumentation is modest, it would be wise to allow for some variation.

Think of the forest instead of the trees for a minute. Those engines were designed, built and even repaired the first few times, long before we really cared about the cost of fuel, or optimizing it's use. Back in the day, leaning was more an exercise in treating the engine well, in not fouling it up with lead deposits, rather than saving every penny in avgas. In the originally approved configurations of these planes, the ability to lean precisely was not designed in.

Add to that, repairs done to the engine along the way. Cylinders are weld repaired, and the precise aerodynamics of the inside of the combustion chamber might not have been maintained, the induction system parts might have been exchanged, the venturi in the carb might have been changed for a different type, or slightly damaged in a small undetected carb fire, the baffles could be imperfect, leading to more condensation of the fuel in the induction of one cylinder than the other. Add this all together, and the effect of leaning of one engine found to be ideal, will likely not be ideal on the next engine.

If leaning at less than 75% power is over done, the immediate effect is negligible. Over a slightly longer term though (many minutes) combustion chambers can get hot spots, valve guides, seats, and the valves themselves can be affected. The cost of repairing a stuck exhaust valve will far exceed the saving in fuel getting you to that point.

If you're renting the plane, the operator has already budgeted the fuel consumption at greater than that obtained by precise leaning. The operator has also budgeted the occasional cylinder repair, but would rather not. Unless the operator directs you otherwise, and the flight manual instructs, yes, lean, and understand why you are doing it, but err to the rich side as much as you can. Fuel is the cheapest coolant available to that engine.

If you are using the same very well instrumented, large engine aircraft over and over, and the owner encourages aggressive leaning, within the limits of the flight and engine manuals, learn to do it. The big engines burn a lot, and there are certainly savings to be had.

I personally do not agree with lean of peak operation, and I have never seen a flight manual, or engine operating manual for a GA aircraft which presented that type of operation as being acceptable. I have certainly seen some Lycoming engine operating manuals which prohibited it. If the owner of the aircraft you are flying, is also the qualified engine maintainer, and they are instructing you to run LOP, and the engine manufacturer does not prohibit it, consider it. Otherwise, if you're running LOP, and you stick a valve, are you willing to pay for the repair?

At 5000' going to full rich would be okay, but getting higher, it's not a good idea. At full rich, the engine is likely to stumble, and startle you and your passengers un-necessarily. I once had a 180 HP Cardinal just about stop when I went to full rich changing tanks at 11,500 feet. If I lean and aircraft with no EGT, I will lean until the first sign of any drop in engine power I can perceive, and enrichen it about 10% mixture control position from there. In 35 years of flying, I am not aware that this has ever created a problem.

Just wondering, why would a valve stick from LOP operation?

LOP itself is a non-issue because by definition the engine is cooler (EGT and CHT) than at peak EGT, and peak EGT is authorised by Lyco at/below 75% of max rated power.

The problem is a general one of excessive CHT combined with high cylinder pressures. There is no real data on what values are damaging in the short term but they are higher than most people think.

GAMI's own tests suggest that detonation needs a CHT of about 500F, which absolutely nobody with half decent instrumentation should ever get anywhere near, but a pilot without instrumentation very well could do so simply by flying at Vx or Vy for 10+ minutes.

For long engine life, the general consensus, such as there is :) is that one should cruise at 65% of max rated power, and peak EGT (or LOP if you want to fly slower, yet maintain full throttle) are perfectly fine at that power setting.

During climb, one cannot get more than 65% above about 12000ft (chart).

My nice-weather IFR cruise setting is FL100, 2400rpm, peak EGT, full throttle, which is about 65% power. For better MPG I go to FL120 and drop down to 2200rpm and 9.5GPH which according to this is about 45% power.

This type of operation produces a very long engine life, with clean cylinders, valves, etc.